Device for the electrolytic protection of a ship&#39;s metal skin against corrosion



SSNACK ET A l. 2,916,429

Dec. 8, 1959 v0 DEVICE FOR THE ELECTROLYTIC PROTECTION OF A SHIPSVMETAL SKIN AGAINST CORROSION 2 Sheets-Sheet 1 Filed Sept. 23. 1957 INVENTORS ATTORNEY 1959 E. VOSSNACK ETAI. 2,916,429

DEVICE FOR Tl-E ELECTROLYTIC PROTECTION 0F A SHIP'S METAL SKIN AGAINST CORROSION Filed Sept. 25. 1957 2 Sheets-Sheet 2 INVENTORS BY W ATTORNEY DEVICE FOR THE ELECTROLYTIC PROTECTION OF A SHIPS METAL SKIN AGAINST CORRO- SION Application September 23, 1957, Serial No. 685,533 6 Claims. (Cl. 204-196) The invention relates to a device for the electrolytic protection against corrosion of the submerged part of a metal ship or of metal parts projecting therefrom, by means of an electric D.C. source, the negative electrode of which is connected to the ships skin, while the positive electrode is connected to a submerged electrode electrically insulated from the ship.

Devices of this type are known. In these cases the positive electrode (anode) is formed by a body (a conductor or a semiconductor) which has been fitted on and insulated from the ships skin. It is a drawback of the known device that the ship has to be docked for mounting, inspection, and possible replacement of this anode or anodes.

Another drawback consists in that it is difiicult to disperse the electric field that is formed in the water surrounding the ship in a uniform way. In any case the skin around and near the anode(s) in question has to be provided with an insulation field in order that the requisite dispersion of the electric field may be attained and no difiiculties owing to excessive local current density may be encountered. The provision and maintenance of this insulation field in itself constitutes a rather unattractive complication.

The invention has for its object to obviate the drawbacks referred to. According to the invention the positive electrode is made as a trailing wire or bundle of such wires, reaching into the water while clear of the submerged part of the ships skin.

In this connection wire is understood to be material of very great slenderness, so that it may also include tape, ribbon, etc.

When the trailing wire or wires according to the invention is (are) payed out from the ships side, from symmetry considerations it is necessary to pay out at least one or more wires on either side of the ship, if the electric field is to be formed uniformly. It has been found that it will generally be sufiicient to pay out one trailing wire from the stern in or near the central plane of symmetry of the ship. in this way a sufiiciently uniform electric field is easily obtained round the whole submerged part of the ship, while the technical drawbacks of parts projecting sideways from the ship are eliminated.

A trailing wire payed out from the stern involves the drawback that it may get entangled in the propeller during the ships manoeuvring, which may cause serious stagnation and damage. According to the invention, however, the trailing Wire may be constructed to have small breaking strength. In that case entanglement of the wire in the propeller can scarcely cause any damage or stagnation. It has been found that for this the breaking strength of the wire must not be more than some hundred pounds, e.g. 600 lbs. (breaking strength of the normal log line: 700 to 900 lbs.). Good conductivity for electric current, however, is to be aimed at, so that the material to be chosen for the wire should have small breaking strength in combination with good conductivity 2,916,429 Patented Dec. 8, 195.9

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for electric current. Consequently, aluminium or an alloy thereof is particularly suitable for the trailing wire or wires in question. Moreover aluminium has the advantage that the trivalency of this metal involves a relatively small loss of weight per ampere and per unit of time during use.

It is naturally possible, in the system of electrolytic protection described above, to reverse the direction of the current periodically, in order to counteract polarization difliculties on the submerged part of the ship. With reference to the drawing, an embodiment of the invention will now be described.

Figure 1 is a diagrammatic perspective drawing of the stern of a ship, equipped with a device for electrolytic protection against corrosion of the submerged part of the ship.

Figure 2 is a diagrammatic plan view of the nature of the electric field in the water upon application of the invention.

In Fig. 1 the ships skin to be protected is denoted by 1. The parts to be protected simultaneously therewith are the rudder post 2, the rudder 3, and the propeller 4. The conductive connection between the propeller and the ships skin is provided by a set of brushes 16, which move along a slip ring mounted on the propeller shaft. This provision is necessary because in general the propeller 4 is electrically insulated from the ships skin, owing to the oil film that has formed between shaft and bearings during the rotation of the shaft.

Mounted on the stern and insulated from the ship is a winch 5, on which annealed aluminium wire 6 with a diameter of about inch has been wound. The wire 6 is payed out, so that during sailing about 45 yards of wire is above the waters surface, while about 10 yards trail through the water. This latter part of the wire will wear away regularly from the end and is supplemented by the unwinding of the winch 5. Immediately behind the winch a drawing knife 7 may be fitted. If the wire gets entangled in the propeller, it will be tightened in the drawing knife 7 and will immediately be cut in the latter.

The soft aluminium of which the wire consists as a rule will not be able to cause much damage to the propeller or other parts of the ship.

As D.C. source a converter has been mounted on the ship, which converter consists of an electric motor 10, supplied from the ships mains and coupled directly with a low-voltage generator 11. This generator 11 supplies a voltage adjustable between 0 and 25 volts and having a maximum current strength of 250 amperes. Voltage and current strength can be read on the voltmeter 12 and the ammeter 13.

The positive electrode of the generator 11 is connected via the conductor 14 to the aluminium wire wound on the which 5, which the negative electrode is connected via the connection 15 to the ships skin.

During the operation of the device a given potential difference is to be maintained between the ship and a submerged reference electrode (e.g. a copper-copper sulphate electrode). The potential of the ship should be between 800 and 850 millivolts in relation to this reference electrode.

If the potential difference exceeds e.g. 900 millivolts, this will result in excessive protection, involving hydrogen evolution and formation of alkali hydroxide, in consequence of which the paint or other protective coatings can be forced off the skin and a scale deposit formed. The current strength in the aluminum wire is therefore adjusted in such a way that the desired potential dilference is obtained.

The potential dilference is regularly measured in different places along the ship's skin by means of the reference electrode hanging in the water, in combination with a potentiometer suitable for this measurement.

The adjustment of the required current strength in the wire is efiected by hand. It is naturally also possible to have this current strength automatically adjusted to the required value by means of a reference electrode (reference electrodes) secured in the ships skin and a suitable controlling device.

In Fig. 2 the submerged end of the wire is denoted by 21 and the lines of force by 22.

What we claim is:

1. Apparatus for the electrolytic protection against corrosion of the sugmerged part of a ships metal skin or of metal parts projecting therefrom, comprising an electric D.C. source the negative electrode of which is connected to the ships skin and the positive electrode of which is connected to a submerged electrode electrically insulated from the ship, said submerged electrode reaching into the water clear of the submerged part of the ships skin and being bare electrically of conductive wire-like means.

2. Apparatus according to claim 1, and further comprising means for paying out said wire-like means from the stern of the ship approximately in the central plane of symmetry thereof.

3. Apparatus according to claim 1 wherein said wirelike means has low breaking strength.

4. Apparatus according to claim 3 wherein the break ing strength is no greater than 600 pounds.

5. Apparatus according to claim 1 wherein said wirelike means comprises aluminium.

6. Apparatus according to claim 1 wherein said wirelike means comprises an aluminium alloy.

References Cited in the file of this patent UNITED STATES PATENTS 2,863,819 Preiser Dec. 9, 1958 FOREIGN PATENTS 13,971 Great Britain Sept. 5, 1890 

